After they spent the money of $15 and $24 respectively, Sally has $x+9 and Susan has $x. Which is at a ratio of 4:3

(x+9)/x=4/3
4x=3x+27
x=27
So after spending the money, Sally has $36 and Susan has $27.
How much did they have to start with? Add $15 to Sally and $24 to Susan.
36+15=51
27+24=51
They had $51 each to start with.